ABSTRACT
So far we have considered only the focusing of paraxial rays of charged particles homogeneous in energy. The true situation is more complex and we should consider rays which are non-paraxial and particles which have a small spread in energy. Particle lenses are subject to the same optical aberrations as photon lenses and the effects can be much worse because it is not possible to use materials of different dispersion to reduce chromatic aberration or to grind surfaces of special forms to reduce spherical and other aberrations.
